The present invention relates to a wireless packet data communication system having a central system and mobile terminals, and in particular, to recovering from a disaster at the central system.
In a communications system having a central system and addressable mobile terminals, there is a forward or downstream channel and a reverse or upstream channel.
On the downstream channel, the central system broadcasts information accessible to, sometimes referred to as heard by, all of the mobile terminals. The downstream broadcast comprises system control messages including control messages at predetermined time intervals and packets of message data respectively addressed to a single mobile terminal (or group of terminals sharing a common address). The downstream broadcast is essentially continuous. The downstream channel may comprise several frequencies, and the central system may use one frequency for its transmission, or may hop among frequencies.
On the upstream channel, the mobile terminals respectively transmit packets of message and system-related data to the central system using a contention protocol wherein if packets collide, retransmission is attempted after an essentially random time interval. The mobile terminals directly communicate only with the central system, not with each other. The upstream channel may comprise several frequencies, and a mobile terminal may use one frequency for its transmission, or may hop among frequencies.
The communications system is designed so that the central system is aware of each mobile terminal able to communicate therewith. When a new mobile terminal enters the geographic area served by the central system, which occurs from time to time, the mobile terminal must register with the central system.
Registration commences when a mobile terminal sends, via the upstream channel, its public identity number to the central system along with a request for a temporary identifier. The public identity number and the request for a temporary identifier are examples of system related data. The central system responds by sending, via the downstream channel, a temporary identifier, which is a randomly chosen number in a predetermined range which has not previously been assigned to any other mobile terminals in communications range of the central system, to the requesting mobile terminal. The temporary identifier is an example of a system control message which is sent as needed, that is, not at a regular interval.
During registration, the central system adds to its routing table an entry comprising the external address of the mobile terminal and the temporary identifier assigned to the mobile terminal. The routing table may also include an upstream frequency for the mobile terminal. It will be appreciated that the routing table changes frequently due to the mobility of the terminals. Since the routing table is stored in one place, the central system is vulnerable to loss of the routing table. The central system uses the temporary identifier as the address for the mobile terminal in subsequent downstream message data packets. The temporary identifier is preferred to other pre-existing identifiers for security and management reasons.
The requesting mobile terminal and the central system then exchange encryption information. After secure communications are established, the mobile terminal sends its secret identity number to the central system. The central system uses the secret identity number to access a stored profile for the mobile terminal containing various subscriber information, including billing information.
The stored profile for the mobile terminal also contains an external address for a subscriber associated with the mobile terminal. The public identity number of the mobile terminal may be its external address. From time to time, external systems send message data packets having the external address of the mobile terminal to the central system, which uses the external address and its stored routing table to obtain the temporary identifier of the mobile terminal. The central system then transmits the message data packets on the downstream channel with the temporary identifier as destination address. The mobile terminal captures and processes data packets addressed to its temporary identifier.
If a problem occurs at the central system, such as loss of its routing table, which affects the essentially continuous transmission on the downstream channel, some mobile terminals may interpret such problem as an error condition and attempt to register anew with the central system. As these mobile terminals attempt to register anew, the throughput of the upstream channel will be reduced, which may cause further problems at the central system. Yet more mobile terminals will perceive an error condition. Eventually, essentially the entire population of mobile terminals will be in contention for the upstream channel while attempting to register anew, so the throughput of the upstream channel will be degraded to a very low value. Consequently, re-registration will take an inordinately long time, and the message data packet throughput of the communications system during such re-registration will become unacceptably low.
Thus, there is a need for a way to register essentially the entire population of terminals, that is, reconstruct the central system's routing table of temporary identifiers, after a disaster at the central system which has obliterated its routing table.